The objective of this project is to develop a pulsed ESR apparatus optimized for the study of nitroxides and other compounds of interest under in vivo conditions. Electron spin resonance (ESR) is a powerful tool for free radical studies and can be useful in biological work is excessive attenuation by water is resolved. The system being developed by BEIP will operate at about one-thirtieth of the standard 9GHz frequency to alleviate the attenuation problem, and will incorporate pulse techniques developed in NMR, as well as other techniques, to compensate for the thirty-fold loss in sensitivity. For several years, nitroxides have been a major focus of research in ROB, NCI because of their importance to radiation biology in general and their potential utility for new photodynamic therapy techniques. DSB provided the PC computer interface to control the high-speed electronics and acquire the data for a FFT plot with Matlab. Initial measurements will be spectroscopic, but imaging will be subsequently undertaken. Achievement of the proposed specifications will greatly facilitate research on nitroxides and other compounds of interest in biological systems. The contribution of DBS/DCRT provided to the project is complete. We have loaned BEIP a PC computer and developed interface software for their instrument. The project will continue with BEIP and ROB/NCI.